1. Field of the Invention
The present invention relates to a receiver and a method of receiving a multi-carrier CDMA signal.
2. Description of the Related Art
A code division multiple access (CDMA) system has been developed through direct sequence spread-spectrum modulation (DS-SS) as a mobile communications system in the coming generation. In the CDMA, signals of respective users in the same carrier are split using different codes and code-multiplexed. Then, the frequency band assigned to the system is used by frequency-multiplexing a plurality of code-multiplexed carrier signals. In this case, a receiving device at a base station has to demodulate CDMA signals of a plurality of signals.
FIG. 1 shows the configuration of the receiver of a conventional multi-carrier CDMA signal.
The configuration shown in FIG. 1 corresponds to one sector and one branch. Each carrier signal is received by a corresponding receiver (RX) 190 (190-1, 190-2, . . . , 190-n), and is processed by filtering, despreading, and demodulation. In a base station by a cellular system, the capacity of the system can be increased using sectors. A sector can be obtained by dividing a cell covered by one base station into further smaller areas. If a cell is a circular area having a base station at the center, it can be radially divided into sections from the base station as the center. Each of the RXs arranged to each sector receives a multi-carrier signal. In the DS-SS receiver, a RAKE receiver is used for synthesizing a multipath. In the RAKE reception at a base station in the CDMA system, signals from a plurality of sectors are RAKE-synthesized so that a hand-over function of switching a sector can be realized without changing the connection when a mobile unit moves between sectors. In order to flexibly correspond to the fluctuation of traffic the mobile unit moving between sectors, the configuration of a system for processing despreading, demodulation, and RAKE-synthesis can be designed as sector-free. A sector-free system refers to a system capable of allowing a base station to process a signal of a mobile unit without considering its movement between sectors as if the mobile unit were in a sector even when it moves between sectors.
Described below are the configuration and operations of a receiver of conventional multi-carrier CDMA signals shown in FIG. 1.
In FIG. 1, signals received from an antenna 1900 are input to respective RXs 1901-1 through 1901-n. Then, they are multiplied by the periodical waves of the frequencies f1 through fn output from station oscillators 1902-1 through 1902-n, and converted into IF signals. After being band-limited by band pass filters 1904-1 through 1904-n, they are detected by quadrature detectors 1905-1 through 1905-n to generate I signals and Q signals. The I signals and Q signals are analog signals, converted into digital signals by A/D converters 1906-1 through 1906-n including an A/D converter 1906A for I signals and A/D converter 1906B for Q signals, and input to fingers 1908-1 through 1908-n through an interface 1907. In the fingers 1908-1 through 1908-n, the receives digital I and Q signals are despread by a despreader 1908A, demodulated by a demodulator 1908B, and output to a synthesis unit 1909. The synthesis unit 1909 RAKE-synthesizes a signal demodulated by each of the fingers 1908-1 through 1908-n, and transmits the signal to a signal discrimination unit (not shown in the attached drawings) described later.
(1) Problems with the Number of Interface Signal Lines
When the processes in and after the despreading process and the demodulating process are performed in a digital process, a signal received by each RX 1901 is A/D converted, and input as a digital signal to a base band processing unit 1911 through the interface 1907. To realize the sector-free configuration in the interface 1907, connections should be established such that all despreaders 1908A (fingers) can select signals from arbitrary sectors, branches (a plurality of independent receiving systems provided for diversity reception using an antenna), and carriers so that they can realize RAKE synthesis. Therefore, the interface 1907 between an RF unit 1910 and the base band processing unit 1911 requires the following number of signal lines.
Number of Signal Lines=number of sectorsxc3x97number of branchesxc3x97number of carriersxc3x97number of A/D bitsxc3x972
where xe2x80x982xe2x80x99 in the last term indicates the number of the lines of I and Q signals.
For example, assuming that the number of sectors is 6, the number of branches is 2, the number of carriers is 4, and the number of A/D bits is 8, the total number of signal lines is 768. Since the frequency of the digital signal obtained after the A/D conversion is high (for example, several ten MHz) in the W-CDMA, a high-speed and large-capacity bus interface 1907 is required, thereby interfering the realization of hardware.
(2) Problems in Responding to Multiple Chip Rates
In the CDMA system, a system using multiple chip rates is investigated as a method for flexibly accommodating information signals at various transmission rates. In this case, if each carrier signal is received by an individual RX 1901, it is necessary to provide the filter 1904, the despreader 1908A, and the demodulator 1908B exclusively for each chip rate.
The present invention aims at providing a circuit configuration requiring a simple and small volume of hardware, and capable of easily responding to multiple chip rates in the receiver of multi-carrier spread-spectrum communications.
The receiver according to the present invention demodulates a spread-spectrum signal modulated into a plurality of carrier frequencies, and includes a detection unit for collectively converting signals obtained by multiplexing multichannel carrier signals into a multiplexed signal of a predetermined frequency band; a carrier demultiplexing unit for performing a carrier demultiplexing process and a frequency shifting process on a signal obtained by the detection unit; and a despreading demodulation unit for performing a despreading process and a demodulating process on a base band signal of each channel obtained in the demultiplexing process and the frequency shifting process.
The receiving method according to the present invention demodulates a spread-spectrum signal modulated into a plurality of carrier frequencies, and includes the steps of (a) collectively converting signals obtained by multiplexing multichannel carrier signals into a multiplexed signal of a predetermined frequency band; (b) performing a carrier demultiplexing process and a frequency shifting process on a signal obtained the step (a); and (c) performing a despreading process and a demodulating process on a base band signal of each channel obtained in the demultiplexing process and the frequency shifting process.
According to the present invention, signals on a plurality of carriers are simultaneously detected, and then a signal of each carrier is extracted later. As a result, the number of signal lines in the interface unit can be reduced between a circuit operating corresponding to an RF band up to the detection unit or a transmission speed of a signal in the IF band and a circuit operating corresponding to the chip rate or a symbol rate in and after the despreading demodulation unit.
Furthermore, since a plurality of carriers can be simultaneously detected and the detected signals can be distributed to a plurality of despreading demodulation units, a sector-free receiver can be easily designed according to the present invention.